1. Field of the Invention
The present invention relates to a lithographic apparatus and a method for manufacturing a device. In particular, it refers to process and alignment corrections of batches of substrates exposed by the lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
One feature of a lithographic apparatus is the precision with which it exposes each resist layer of the same substrate such that all layers on the same substrate are aligned. In order to align the layers of a substrate, the table that holds the substrate must be controlled such that the substrate itself is aligned in the same place for every exposure. The same set-up is used for several batches of substrates, and so the alignment of a batch of substrates must also be the same for each substrate and each layer of each substrate in order for a specific exposure set-up to expose all of the layers of all of the substrates precisely in the same manner.
The way that alignment is carried out is that each layer on a substrate includes a number of sets of alignment marks that define the position and shape of the substrate. This position and shape are described by a mathematical model. Generally, more alignment marks are used than are needed to describe the model and so there is residual mark data. Alignment of the substrate layers is successful when a layer being applied for exposure (the “exposure layer”) is positioned accurately on top of previous layers. When alignment strategy is not successful, overlay errors occur and less satisfactory exposed patterns on the substrates are created, which may make the resultant IC, etc., not work as well, or the substrate and all its layers may have to be redone if the overlay error is serious enough. This reduces throughput of satisfactorily exposed substrates.
During the alignment of a substrate, the possibility exists to choose from different mark types, different beam colors (or wavelengths) and different diffraction orders to be diffracted from the alignment mark. All these different possibilities give different misalignment results. During measurement of the marks, a decision is made as to which mark type, how many marks, which beam color, and which diffraction order to use.
Overlay measurement or metrology occurs outside of a lithographic apparatus in a dedicated overlay metrology tool. After the substrates are exposed and each, a predetermined number, or all of the resist layers on the substrate are developed, a batch of substrates is sent to the overlay metrology tool and a number of the substrates has their overlay measured. There are several ways to measure overlay errors. Most involve the use of a set of overlay targets or marks that are present in the resist layer and in a previous layer and the relative positioning of the two is measured by reflecting/diffracting a radiation beam from the superposed marks. These overlay marks may be separate from alignment marks, or the same marks may be used for both purposes. If two layers containing marks are perfectly aligned, when an overlay metrology beam is directed onto the mark, the diffracted beam will show no misalignment or overlay of the marks. The marks may include a grating or a periodic array of structures. Misalignment of superposed marks causes variations to the diffracted beam. The extent of the misalignment of the marks (e.g. the gratings) causes an equivalent extent of variation to the diffracted beam. The calculated misalignment values may be stored in a library and future misalignment values compared with these library values to determine the extent of the overlay error. When the library contains enough data, misalignment values can be lined to overlay errors without having to recalculate the image created by the diffraction beam, as the library will contain all the data required.
In the state of the art, the same number of alignment marks (e.g. 16 pairs) is used for each batch of substrates. To compensate for fluctuation in alignment, there are occasional alignment adjustments that are made on the substrates so that the alignment marks are in alignment with alignment marks of previous layers. “Process corrections” (of which alignment corrections form part) are made bespoke to the alignment model for each batch. For example, if a specific lithographic apparatus has a tendency to cause a slight clockwise rotation during exposure of the substrates, the alignment model for that apparatus may incorporate a counterclockwise rotation. The process correction is based on previous batches that have been exposed by that lithographic apparatus.
Problems with the state of the art arise when some substrates have different numbers of alignment marks or alignment marks in different places from other substrates, or if a misalignment is so extreme that the alignment information available from the alignment marks is insufficient to determine the process correction or alignment correction required. Process corrections are dependent on the alignment of the same marks and so if an alignment mark is missing or in a different place, the adjustment controller does not have the correct input information and may therefore be made in respect to the wrong marks. For example, if all previous batches of substrates had 16 pairs of marks and a substrate is included that only has 14 pairs of marks, the controller tries to make its calculations to adjust the position of the substrate with an algorithm made for 16 pairs, but with the input of only 14 pairs. Process corrections are therefore not only difficult to calculate, but a correction may be made that in fact makes the alignment worse because the input information was wrong.